Prior to the development of optical communication systems, substantially all information transmission was accomplished through the use of electrical cable systems, radio frequency transmission and, more recently, point-to-point microwave links. For certain applications, cable systems represent the optimal solution, in view of their capacity to carry large numbers of channels and their immunity to interference, as well as because of the fact that they do not occupy the limited on-the-air radio frequency spectrum.
Thus, over the years, extensive cable systems carrying telephone calls, data, telex and television have been developed. With the increasing development of optical communication systems, many of these cable networks are being replaced and/or upgraded with optical fiber cable systems. Optical systems are particularly desirable in view of their ability to carry greater numbers of channels, their relatively low failure rates, and their ability to eliminate the need for large numbers of repeaters in cascade, which, in principle, should result in reduced distortion. Today, the primary use of such systems is in digital voice and data transmission systems.
Despite the above advantages, the application of fiber optic techniques presents several challenges to certain major end-users. While transmission over the fiber optic line is often vastly superior both in terms of span length between repeaters and the number of independent channels which may be carried by a single fiber, the generation of a highly linearly amplitude-modulated (AM) light signal presents substantial obstacles. These problems may be particularly serious where it is desired to integrate a fiber optic span into an existing cable system. If the existing system is a digital system, the quality of the modulation of the system is of relatively minor importance in view of the fact that such systems function well in relatively high noise and high distortion environments. Similarly, in the case of frequency modulated (FM) systems, the wide bandwidth of the signal, the redundancy of the sidebands and the resultant natural immunity of FM systems to noise again results in generally acceptable performance. However, in the case of high fidelity amplitude modulated systems, substantial problems are presented.
One particular application in which it is desired to maintain large parts of an existing network is in the cable television industry. Here, the large number of individual subscriber connections and extensive local area cables makes complete replacement of the system undesirable. Moreover, the large number of existing television sets and the necessity of the system to accommodate equipment marketed for off-the-air television reception necessitates that at least some portion of local signal distribution be in a conventional AM radio frequency format.
Despite the advantages, as yet, there has been little penetration of the cable television industry by fiber optic systems due to the difficulty of meeting minimum standards which the industry has proposed, despite substantial research efforts aimed at providing high quality optical amplitude modulation.
In the event that one wishes to put amplitude modulation onto a carrier, be it an optical carrier or otherwise, it is necessary to modulate the intensity of the carrier linearly in proportion to the information which one wishes to transmit. Generally, such linear modulation may be defined as multiplication by a constant scaling factor and/or the addition of a DC bias level. Such operations are linear, and any device which performs such operations will have an output with frequency components identical to its input frequency components. However, in the event that there are non-linearities in the system, the output will also include components which represent multiple sums and differences of the input frequency components.
In principle, there are a number of ways of obtaining a modulated laser light beam. In the case of semiconductor diode lasers, the most direct modulation method is to apply a constant electrical input current to bias the diode above its lasing threshold and then to add a variable current proportional to the desired information signal. If the light output were to vary linearly with the input current above threshold, then the optical signal would be a high-fidelity replica of the original information. Unfortunately for many end-uses, it is very rare to find a semiconductor laser that possesses a degree of linearity that is sufficient. As reported recently in the Proceedings of the 1989 Annual Meeting National Cable Television Association.sup.1 the cost of these carefully selected lasers prohibits their widespread use at present.
1 James A. Chiddix, "Fiber Backbone--Multi-Channel AM Video Trunking", pp 246-253, May 1989.
Other alternatives include modulating the intensity of a constant source, for example, through the use of a Kerr or Pockels cell. Unfortunately, such modulation techniques are inherently nonlinear and exhibit relatively good linearity only when the amplitude of the modulation is impractically small. Moreover, in the event of such relatively small dynamic range in modulation, other factors such as source and receiver noise and the like act to effectively overcome any gains in fidelity achieved as a result of using a very small part of the device characteristic, albeit relatively linear.
In a paper prepared for the Air Force Office of Scientific Research entitled Use of Predistortion to Reduce Intermodulation Distortion in Optical Fiber Communication Sources.sup.2, Larson and Smith proposed compensating for the nonlinearity of a diode by predistorting the signal input to the light-emitting diode. In accordance with this technique, an optical receiver and a light-emitting diode transmitter are constructed to measure the intensity of light output by the light-emitting diode a against its input current.
2 Report AFOSR-TR-79-0904 May 1979.
A polynomial describing the dynamic characteristic is then developed and used to predict the intermodulation distortion as a function of the percentage of modulation. A compensating network is then developed. Reduction of intermodulation distortion products in the range of 6-15 decibels has been noted using this technique. While such improvements certainly are significant, they do not nearly approach the degree of suppression necessary to achieve a high quality amplitude-modulated optical cable network.
Furthermore, such precompensation is possible only in cases where the non-linearities of the light emitting source are well-defined, predictable and non-varying. This is not generally the case for high power semiconductor devices.
A more effective approach was proposed in an article entitled Linear Interferometric Modulators in Ti: Li Nb O.sub.3 by Bulmer and Burns in the Journal of Lightwave Technology, Volume LT-2, No. 4, of August, 1984. In accordance with this system, a Mach-Zehnder interferometric modulator with asymmetric arms is used to achieve greater linearity. Generally, the device described in this article comprised a two-armed device in which one of the arms is given a DC bias which results in an intrinsic phase bias between coherent light from a single source which exits the two arms, of approximately 90 degrees. While such an arrangement does have the effect of substantially suppressing second and fourth intermodulation distortion products, the substantially unaffected third order distortion products do not render the system adequate for high quality, relatively high power and high capacity optical cable information distribution systems.
In a paper entitled Reduction of Intermodulation Distortion in Interferometric Optical Modulators presented by Johnson and Roussell at the IEEE/LEOS Meeting, held at Santa Clara, Calif. in November of 1988, a dual polarization technique was proposed for reducing intermodulation distortion in waveguide interferometric modulators. Here, an interferometric modulator supporting single transverse electric and transverse magnetic modes was driven by a modulating signal. Because there is a three-fold difference in the voltage sensitivity between the TE and TM modes, an input light polarization angle could be found that would suppress selectively the dominant cubic term of intermodulation distortion.
However, such an approach suffers from additional problems such as the simultaneous processing of two different polarization components which would impose additional problems in manufacture and operation of the device. In addition, interference effects arising from interactions between scattered portions of the two polarization components within the optical fiber transmission line will give rise to unwanted intermodulation products and/or compression-expansion terms in the output TV signal.